linux-kernel - Re: [PATCH 01/10] Add support for GCC-4.5's __builtin

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Message-ID: <4AAE63E7.90909@caviumnetworks.com>
Date:	Mon, 14 Sep 2009 08:40:23 -0700
From:	David Daney <ddaney@...iumnetworks.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
CC:	Michael Buesch <mb@...sch.de>, linuxppc-dev@...ts.ozlabs.org,
	torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
	linux-mips@...ux-mips.org,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linuxppc-dev@...abs.org, Paul Mackerras <paulus@...ba.org>,
	"H. Peter Anvin" <hpa@...or.com>, linux-s390@...r.kernel.org,
	linux-am33-list@...hat.com, Helge Deller <deller@....de>,
	x86@...nel.org, Ingo Molnar <mingo@...hat.com>,
	Mike Frysinger <vapier@...too.org>,
	Ivan Kokshaysky <ink@...assic.park.msu.ru>,
	uclinux-dist-devel@...ckfin.uclinux.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Henderson <rth@...ddle.net>,
	Haavard Skinnemoen <hskinnemoen@...el.com>,
	linux-parisc@...r.kernel.org, linux-kernel@...r.kernel.org,
	ralf@...ux-mips.org, Kyle McMartin <kyle@...artin.ca>,
	linux-alpha@...r.kernel.org,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	linux390@...ibm.com,
	Koichi Yasutake <yasutake.koichi@...panasonic.com>
Subject: Re: [PATCH 01/10] Add support for GCC-4.5's __builtin_unreachable()
 	to compiler.h

Geert Uytterhoeven wrote:
> On Fri, Sep 11, 2009 at 17:58, David Daney<ddaney@...iumnetworks.com> wrote:
>> Michael Buesch wrote:
>>> On Friday 11 September 2009 01:56:42 David Daney wrote:
>>>> +/* Unreachable code */
>>>> +#ifndef unreachable
>>>> +# define unreachable() do { for (;;) ; } while (0)
>>>> +#endif
>>> # define unreachable() do { } while (1)
>>>
>>> ? :)
>> Clearly I was not thinking clearly when I wrote that part.  RTH noted the
>> same thing.  I will fix it.
> 
> However, people are so used to seeing the `do { } while (0)' idiom,
> that they might miss
> there's a `1' here, not a `0'.
> 
> So perhaps it's better to use plain `for (;;)' for infinite loops?
> 

I don't think so.  The only valid token that can follow 'do { } while 
(1)' is ';', any statement may follow 'for (;;)', so there is a greater 
possibility to silently screw things up with the for(;;) form.

David Daney

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